Slack Block Kit Builder — Helper for AI-Generated Messages

Quick Use

1. pip install slack-bolt
2. Drop md_to_blocks() into your bot module
3. Pipe LLM markdown straight into chat.postMessage(blocks=md_to_blocks(text))

Intro

This is a small Python helper that takes plain markdown from an LLM and produces valid Slack Block Kit JSON — header, section, divider, fields, buttons, image — automatically chunking text past Slack's 3001-char per-block limit. Slack messages without Block Kit look terrible; messages with Block Kit require fragile hand-built JSON. This bridges them. Best for: AI bots posting summaries, alerts, daily digests, and reports to Slack channels. Works with: Slack Bolt SDK (Python/JS), Slack chat.postMessage REST. Setup time: 5 minutes.

Helper code

from typing import Iterable

def md_to_blocks(text: str) -> list[dict]:
    blocks = []
    for raw in text.split("\n\n"):
        chunk = raw.strip()
        if not chunk:
            continue
        if chunk.startswith("# "):
            blocks.append({"type": "header", "text": {"type": "plain_text", "text": chunk[2:].strip()[:150]}})
        elif chunk == "---":
            blocks.append({"type": "divider"})
        else:
            for piece in _split_3000(chunk):
                blocks.append({"type": "section", "text": {"type": "mrkdwn", "text": piece}})
    return blocks

def _split_3000(s: str, limit: int = 2900) -> Iterable[str]:
    while len(s) > limit:
        cut = s.rfind("\n", 0, limit)
        if cut == -1:
            cut = s.rfind(" ", 0, limit) or limit
        yield s[:cut]
        s = s[cut:].lstrip()
    if s:
        yield s

def add_actions(blocks: list[dict], buttons: list[tuple[str, str, str]]) -> list[dict]:
    if not buttons:
        return blocks
    blocks.append({
        "type": "actions",
        "elements": [
            {"type": "button", "text": {"type": "plain_text", "text": label},
             "action_id": action_id, "url": url}
            for label, action_id, url in buttons
        ],
    })
    return blocks

Usage with Bolt

from slack_bolt import App
from slack_bolt.adapter.socket_mode import SocketModeHandler
import os

app = App(token=os.environ["SLACK_BOT_TOKEN"])
llm_text = '''# Daily AI Digest

*5 new MCP servers* shipped on TokRepo today.

- Stripe MCP — payments
- Datadog MCP — observability
- Linear MCP — issue tracker

---

Top reaction: stripe-mcp got 47 stars in 6 hours.'''

blocks = md_to_blocks(llm_text)
blocks = add_actions(blocks, [("Open TokRepo", "open_tokrepo", "https://tokrepo.com/explore")])

app.client.chat_postMessage(channel="#ai-digest", blocks=blocks, text="Daily AI Digest")

Slack Block Kit limits (cheat sheet)

FAQ

Q: Why mrkdwn instead of markdown? A: Slack's mrkdwn is a subset — *bold* (one asterisk, not two), _italic_, <url|label> for links, no headings, no tables. The helper outputs mrkdwn-flavored text. Hand-converting LLM markdown is the part this saves.

Q: Does the LLM need to know about Block Kit? A: No — that's the point. Prompt it for normal markdown. The helper handles the conversion. Hand the LLM Block Kit JSON in the prompt and outputs become brittle and verbose.

Q: What if the message is over 50 blocks? A: Slack rejects with invalid_blocks error. Send as a thread — first message gets the first 50 blocks, then chat.postMessage with thread_ts for follow-ups. Or split into multiple top-level messages with day/section headers.

Source & Thanks

Pattern compiled from Slack Block Kit + Bolt SDK.

Bolt MIT-licensed, helper Apache-2.0.